Screw pumps



y 1970 M. B. SENNET ETAL 3,519,375

SCREWTUMPS Filed June 18, 1968 l2 l0 l4 1 Z 26 24 29 //{///1 /l% 3 a w I 6 4 +2 22 a f y n FIG FIG. 2

INVENTORS MORGAN a. SENNE JOHN E. GRIFFIT ATTORNEYS United States Patent 3,519,375 SCREW PUMPS Morgan B. Sennet, Erwinna, and John E. Griffiths, Morrisville, Pa., assignors t0 DeLaval Turbine Inc., Trenton, N.J., a corporation of Delaware Filed June 18, 1968, Ser. No. 737,891 Int. Cl. F04c 1/10, 5/00 US. Cl. 413-179 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The present invention is particularly applicable to both positive and non-positive screw pumps of the type described in the Montelius Patents 1,698,802, 1,821,523 and 1,965,557, dated respectively, Jan. 1929, Sept. 1, 1931 and July 3, 1934, and Sennet Patents 2,693,762 and 2,693,763, dated Nov. 9, 1954. It will be evident that the invention is, however, also applicable to other types of screw pumps in connection with which generally similar problems arise.

Referring particularly to positive screw pumps, these theoretically would have continuous delivery and, in fact, in practice substantially continuous delivery does take place in the sense of displacement of volume per unit time assuming an incompressible liquid is being pumped. Compared with other pumps these screw pumps operate very silently. However, superimposed on this continuous delivery are pulsations which may be particularly violent (in the sense of absolute magnitude) in the case of highspeed pumps operating to deliver at high pressures and therefore involving very large power delivery. The pulsations produced are in the nature of high intensity sound waves (though representing power quite small as a proportion of delivered power), which are propagated through the liquid being delivered, to some extent, and to and through the structure of the pump and its associated piping, etc. Multiple causes apparently conribute to the production of these pulsations and contribute to different degrees in dependence upon the circumstances surrounding operation.

One of the causes of pulsation may be readily recognized. Positive screw pumps contain one or more closures in the sense of boundaries of closed chambers which effectively travel axially during operation. 'Ihese closed chambers open successively to the outlet. Since running clearances must necessarily be provided between the component screws and between the screws and their housing, examination of these will make clear that the resistance to backflow of the pumped liquid varies during a cycle of screw rotation. For example, assume a closed chamber which is approaching but has not yet opened to the outlet. The pressure therein will at this time lie somewhere between the inlet and the outlet pressures of the pump, deviating from a theoretical inlet pressure due to the fact that backflow has occurred to some extent during the progress of this chamber toward the outlet. Nevertheless, prior to opening to the outlet the pressure will ordinarily be considerably less than the outlet pressure, particularly if the clearances are small and the liquid being pumped relatively viscous.

The closed chamber just described is suddenly opened to the outlet during operation. This means that a sudden transient pressure change will occur involving a sharp pressure drop beyond the opening, a momentary drop of forward flow velocity, and then an acceleration involving increase of pressure. If the liquid was completely incompressible, and if completely tight closures were provided, there might theoretically be no source for pulsation at this time. But liquids are compressible and, in fact, this is evidenced merely by the fact that they will transmit sound waves. Solely due to inherent compressibility, it is evident that the sudden pressure changes would give a pulsation having, in particular, a steep wave front. This situation is made all the worse by the fact that such pumps are ordinarily used to handle oils containing volatile constituents and dissolved gases, and their content of gases at low pressures is increased in many cases by cavitation at the pump inlet producing separation of the gas or volatile constituents in bubble form. Accordingly, the sudden transient pressure changes which have been described may produce not only what might be considered a true sound wave but an additional shock or disturbance of compression of the separated gas or volatile constituents.

Another factor which may be involved to a greater or lesser degree is that of sudden change at the same time of the resistance to backflow. It will be evident that the flow path backwardly from the outlet to the inlet incorporates successive restrictions which are provided at the theoretical closure points of the chambers, actual possibility for flow resulting from clearances. The number of restrictions is suddenly changed at the time a chamber opens to the outlet, there being a sharp shift of pressure relationships along the closure lines and surfaces resulting, necessarily, in a change of rate of backward flow. Different pressures are also applied to different parts of the rotors as the phase relationship changes during a cycle.

All of the foregoing, and other causes as well, result in the production of pulsations. As already indicated, these from the standpoint of change of volume of flow percentagewise are merely small superpositions on a theoretically constant fiow rate. But measured in terms of sound energy the pulsations may be of very high noise level and productive of not only noisy but actual damaging vibrations in piping and other structures associated with the pump. Involving shock aspects, furthermore, the noise spectrum is very broad. The fundamental frequency is related to the frequency of rotation of the screws. The usual screw arrangement which is used and which has many advantages is that of a single power screw having two threads meshing with a pair of idler screws each having two threads. In the case of such a pump, the fundamental frequency of the pulsations is twice that of the frequency of revolutions of the power rotor or screw. This would be as expected from consideration of the events occurring during a cycle of operation, there being two traveling chambers opened to the exit during each revolution of the power screw.

Structure-borne noise is a further problem. It may be recognized that reduction of this noise could be effected by providing kinematically perfect screws. If merely simple gearing was involved this would mean balancing of the screws and uniformly constant motion. Balance is not a problem; but securing uniform rotation of the screws is a difficult matter considering the long lead, the shapes of the intermeshing threads, the existence of multiple threads on the screws, and the methods used for generating the threads. This problem is further complicated because the idler screws are not supported in bearings, so that their positions with respect to the power screw are determined by the bores which, in themselves, are difiicult to align precisely. A further complication is involved in that the meshing conditions are related to the lengths of the threads. The number of threads in contact through a revolution varies. For example, in the case of a conventional three-screw set with a meshing length of three times the diameter of an idler there are, twice per revolution, four points in theoretical contact for a small angular period and three points of theoretical contact for a long period. Problems of this type might theoretically be solved by the utilization of screws, particularly idlers, which have resilience and effect damping of pulsations transmitted thereto during operation. However, this property of resilience itself involves deflections and distortions which, though minute, are not always acceptable particularly for operations under extreme pressures.

In accordance with the present invention, the advantage of using substantially rigid metal screws is retained but plastic or other material is used as part of the structure to take advantage of its resilient or sound-deadening properties.

A better understanding of the invention may be had by considering the probable origin of the structure-borne noise, though this is diflicult to recognize with certainty. It has been found that in screw pumps of the type exemplified in the above-mentioned patents it is advantageous not to try to mount the idler screws in shaft bearings nor to try to time them with respect to the power screw by timing gearing. Rather, they are free to rotate in their housing bores as bearings and their timing is determined by their mesh with the power screw. But even with the utmost care in their manufacture and due to necessary running clearances they are inevitably loose though to only a minute degree and cannot be made so perfect as to have the same relationships to the power screw (itself inevitably imperfect) throughout a complete revolution. As a result, the pulsations heretofore referred to as inherent in operation produce vibrations of the idlers both transversely of their axes and longitudinally. The former transmit vibrations to the housing at the walls of the bores and to the power screw. The latter transmit vibrations to the threads of the power screw. These vibrations are of an impact or hammering type.

Viewing the power screw and one idler screw as gears, it can be accepted that the only surfaces in contact are (a) the cylindrical exterior of an idler screw and the cylindrical root of the cooperating power screw, both having equal diameters and rolling on each other and (b) the land of an idler and the mating land portion of the power screw thread flank, this last land being near the thread root. All of the other thread surfaces may be realistically considered to have clearance. In other words, running clearances exist between the outer cylindrical surface of the power screw and the root cylindrical surface of the idler screw, and also between remaining portions of the thread flanks of both screws and the outer edges of the threads, including the outer edges of the power screw threads.

To summarize the last from the standpoint of power transmission, motion is transmitted from only the inner portion of the flank of a power screw thread to the land at the edge of an idler screw thread for each thread pair.

SUMMARY OF THE INVENTION In accordance with the present invention, a resilient plastic insert, or an effective equivalent, is provided at the power-transmitting portion of each power screw thread, with the result that there is a minimizing of the portion of the screw assembly which is non-metallic, or different from the major metal of a screw, the idlers being, preferably, uniformly metallic throughout. While the invention may be applied by making the driven lands of the idlers of special material, the power screw being uniformly metallic throughout, structurally this is more difiicult to accomplish and it is preferred to provide inserts in the power 4 screw. Grooves for reception of inserts may be readily provided in the power screw and after the insertions are made machining of the inserts along with final machining of the power screw may be achieved, whether the machining is by milling, grinding, scraping, lapping, or otherwise.

The vibrations which occur are well cushioned, and slight yielding of the plastic when it is used will provide for smooth and uniform running. The resilience of p1astic minimizes the impacts between the screws.

It may be noted that the structure-borne noise is the source of air-borne noise, through the housing and mounting, so that reduction of the former also reduces the latter.

The objects of the invention have to do generally with the attainment of the indicated objectives, and these objects as well as others relating to matters of composition and the like will become apparent from the following description, read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an axial section taken through a screw pump provided in accordance with the invention; and

FIG. 2 is a transverse section taken on the plane indicated at 22 in FIG. 1 through one of the pump units.

DESCRIPTION OF THE PREFERRED EMBODIMENT The pump arrangement is generally conventional, but, as described hereafter, at least the power screw is modified in accordance with the present invention. Reference will first be made to the mechanical aspects of what is shown.

The numeral 2 indicates generally the casing of the pump which comprises a pair of symmetrical units (though these need not necessarily be the same). As illustrated, the pump is of the positive type in which each unit comprises a power screw containing two threads and a pair of idler screws, each containing two threads. The housing is closed at one end by a cover 4 and at its other end by a cover 6 which is provided with a conventional bearing and packing arrangement for the drive shaft 8 which has formed integrally therewith the power screws; 12 and 14 rotating in a central bore of the inner housing 10. A pair of idlers 16 mesh in usual fashion with the power screw 12, and a similar pair of idlers 18 mesh with the power screw 14. These idlers rotate in the usual side bores of the housing 10. As so far described the arrangement is conventional.

The inlet is at the center of the two units which provide discharge at 24 and 26, respectively. SuITOunding the inner housing is the chamber 28 which receives flow from both of the units. The delivery connection is shown at 32.

The present invention involves modification of the power screw as will now be described.

Grooves 38 are machined into the power screw, these grooves being formed in the region of meeting of the cylindrical root of the power screw and each of the driving surfaces of its thread flanks. Inserted into each such groove is the plastic insert 40 each of which, as will be evident, will extend helically about the power screw axis. Upon finishing, the portion 42 of each insert defines the power-transmitting portion of the flank of a thread.

For simplification of description, plastic inserts will be first described, with the understanding that discussions of these apply also to the use of other materials hereafter referred to.

A considerable variety of plastics may be used to provide the inserts.

One of the highly desirable plastics is the dimensionally-stable acetal (polymerized formaldehyde) plastic material sold under the trademark Delrin produced by E. I. du Pont de Nemours and Company. This material may be machined to close tolerances and may be used under conditions when the temperature does not rise above 200 F. This material shows very low water absorption, and hence may be used for the handling of aqueous liquids or liquids which contain water. It has high compressive and flexural strength. The material is highly resilient, and has excellent elastic properties in both compression and flexure, and thus particularly provides damping of vibrations. As a result of its resiliency the shock waves produced by the operations as discussed above are well-absorbed, and in particular, transmission of shocks by the idlers to the power screw is minimized.

Another type of plastic which is useful is laminated phenolic resin of the type in which cloth or fabric laminations (e.g., of linen) are embedded in a phenolic resin. The phenolic resin here referred to is of the conventional phenol-formaldehyde type. This type of plastic has, for the purposes involved here, properties quite similar to those of the Delrin referred to above, though, since there is a substantial absorbability of water, screws utilizing this plastic are desirable not used when water is present in the liquid being pumped.

Generally similar to the last is the type of plastic consisting of fabric (linen) layers embedded in an epoxy resin.

A fourth type of plastic which is highly satisfactory and exhibits properties similar to those already described is what is known as Fiberglas consisting of an epoxy resin filled with glass cloth or other glass fibre array. This has a lower water absorbability and hence may be used when water is present in the fluid being pumped.

Still another type of plastic which has special advantages is glass fibre or fabric filled polytetrafluoroethylene, the resin here being commercially known as Teflon. Besides having the advantageous characteristics already described, this plastic exhibits a very low coefficient of friction, and hence is particularly usable where the liquid being pumped has poor film strength. Equally usable is Teflon impregnated with powdered metals such as bronzes.

The so-called rigid urethane resins may also be used.

While there have been indicated various plastics which may be used, it will be evident that these constitute merely examples of other plastics having similar characteristics of resilience, shape stability, etc., which are, or may become, available.

The term plastic is used herein in the modern sense of meaning a synthetic resin, either thermoplastic or thermosetting, capable of forming shape-retaining bodies such as rods which may be machined to form the inserts, the machining involving milling, turning, shaving and/or grinding as may be desirable for the particular screw finishes which are desired. These machining techniques are those commonly used for the production of metal screws and may be applied to the types of plastics here involved.

In general, the plastic used for the inserts should exhibit the following properties:

Low absorbability of the liquid to be pumped and minimal dimensional change with such absorption as does occur.

Temperature stability in the range of temperatures which may be encountered in use, including such temperatures as may result from normally expected friction.

Tensile strength upwards of 5,000 pounds per square inch.

Compressive strength upwards of about 18,000 pounds per square inch.

Flexural strength desirably upwards of about 12,000 pounds per square inch, though considerably lower flexure strength may be tolerated.

Impact strength upwards of 0.55 foot-pounds per inch of notch (according to the Izod test).

Low thermal expansion when the pump is to be used through wide ranges of temperature.

Good machining properties to make possible high dimensional accuracy.

In the formation of the power screws, they may be initially machined to dimensions suitable for final machining. In this initial machining the grooves may be formed for reception of the plastic inserts.

The plastic inserts may be preformed by machining operations and then inserted in the grooves. But where injection molding is possible, this is desirable, the injection being into the grooves with provision of bounding walls of approximately the shape of the final external surfaces of the inserts. Bonding of the inserts into the grooves is generally unnecessary if force fits are provided, particularly if there is slight undercutting of the grooves to prevent removal of the inserts, such undercutting being provided when injection molding is used. In any event, in operation, forces exerted on the inserts would be such as to seat them rather than to induce their movements out of the grooves.

Following insertion, finished machining of the metal and plastic portions of the screws may be accomplished in usual fashion with the end result being dimensional accuracy.

As will be noted from what is described, the plastic inserts provide parts of the cylindrical surfaces of the power screws and only limited parts of the thread flanks. This latter is involved since only the root portions of the flanks actually theoretically engage the idler lands. The outer portions of the power screw flanks, even on the driving side, have running clearances, as do also the opposite trailing flanks of the power screw threads, so that plastic in these regions is not required. The resilience of the inserts takes care not only of shock transmissions but also of deviations from theoretical accuracy of the surfaces involved, including such minor variations of pitch as may be residual after final machining. With properly designed screws, the forces which must be transmitted by the plastic inserts are not large. As is well known, the idlers are designed for good hydraulic balance, and theoretically little rotational force need be transmitted to them, the major work involved being that of the power screws against the liquid being pumped.

In the use of plastics as described above, the suppression of noise is probably primarily due to the resilience of the plastic. However, other materials may be used for the inserts in which, in some instances, similar effects are produced due to resilience, while in other instances the desired results are secured by damping due to inhomogeneity secured, in some instances, by layering.

As one example, the plastic may be replaced by a sintered metal containing copper and lead either alone or with inclusion of molybdenum disulphide or phosphorus. Such a material in powdered form may be introduced into the grooves under very high pressure and then sintered at elevated temperature. The result is a somewhat porous material having a relatively low capability of transmitting vibrations, with an end result similar to that of a plastic insert.

Alternatively, even hard inserts may be used. Outstanding are the use of manganese bronze inserts which, though hard and dense have sound-deadening properties. In part, this may be the result of the existence of an interface between two materials, the insert and the screw body, which have very different elastic properties. Brazing alloys can be used instead of manganese bronze.

The matter of inhomogeneity and production of interfaces may also be secured by using laminated materials such as may be produced by the securing of thin strips of harder metals such as bronzes by soldering. Similar laminations may be produced by electrodeposition of alternately different metals, prevention of deposition in the regions not desired being produced by masking.

In fact, even very hard materials may be used in the grooves, such as carbides, Stellite, or the like. Due to the differences in materials sound deadening results. Such 7 hard materials may be held in place by the use of brazing as in the formation of carbide or Stellite tipped cutting tools. The use of metallic inserts is advantageous in situations in which plastics may have deficiencies, as where high temperatures of operation exist or where due to the liquid being pumped various plastics may not be usable because of chemical attack or absorption of liquid constituents.

Summarizing the aspects of the invention, it involves the advantages of utilization of strong, accurately machinable metal screws, the plastic or other inserts being utilized only in those regions in which their resilient or other damping characteristics are advantageously used. The plastic or other inserts are compatible with the otherwise metallic structure. These inserts may be used in screws of both positive and non-positive pumps, damping also being useful in the latter though its advantages are outstanding in the former.

It will be evident that various modifications may be made without departing from the scope of the invention as defined in the following claims.

We claim:

1. A screw pump comprising a power screw having at least one thread and at least one idler screw meshing with the power screw and having at least one thread, and a housing having intersecting cylindrical bores mounting said screws for rotation, the power screw having at least one convex thread surface a helical portion of which drives an outer helical edge portion of a thread of said idler screw, at least one of said portions being formed of a shape-retaining hard machinable material different from that of the major body of its screw, the other of said portions being of the material of the major body of its screw.

2. A screw pump according to claim 1 in which the first mentioned material forms said portion of the convex thread surface of the power screw.

3. A screw pump according to claim 2 in which the portion of the convex thread surface of the power screw formed by the first mentioned material is adjacent to the root portion of the thread.

4. A screw pump according to claim 2 in which the first mentioned material provides an insert located in a helical groove in the power screw.

5. A screw pump according to claim 3 in which the first mentioned material provides an insert located in a helical groove in the power screw.

6. A screw pump according to claim 1 in which the first mentioned material is a resilient shape-retaining machinable plastic material.

7. A screw pump according to claim 2 in which the first mentioned material is a resilient shape-retaining machinable plastic material.

References Cited UNITED STATES PATENTS 1,821,523 9/1931 Montelius 103-128 2,380,752 7/1945 Grieb 230-141 2,754,050 7/ 1956 Wellington 230-441 FOREIGN PATENTS 757,322 10/ 1933 France.

WILLIAM L. FREEH, Primary Examiner W. J. GOODLIN, Assistant Examiner US. Cl. X.R. 418-497 

